Embodiments of the present invention relate generally to diagnostic imaging, and more particularly to a method and a system for operating an electron beam system in a wide dynamic range of emission.
Computed tomography (CT) finds wide application in fields such as clinical diagnosis, industrial inspection and security screening. Several CT systems have been developed, for example, for detecting breast cancer, diagnosing cardiovascular diseases, performing CT fluoroscopy and airport luggage inspection. CT systems require a large number of projection images from a wide range of viewing angles for high quality image reconstruction. Additionally, the CT systems may also need to control the electron beam intensity of the X-rays for reducing patient dose while still achieving desired imaging quality.
To that end, conventional CT systems employ devices such as X-ray tubes having controlled filament heating for electron beam emission control. Conventional filament heating, however, is a slow process of the order of tens of milliseconds, thus preventing its usage in applications where faster electron beam emission control, such as of the order of tens of microseconds, is desirable. The X-ray tubes may further include control means such as an electrostatic grid and/or a magnetic assembly to control the electron beam current. Rapid changes in the electron beam current in such an X-ray tube, however, prevent proper positioning and focusing of the electron beam on a target object. Particularly, modulation of the electron beam current from 0 percent to 100 percent of the electron beam intensity causes repulsion of electrons among one another due to changes in space charge force. The changes in the space charge force further affect the electro-magnetic focusing and deflection of the electron beam in the X-ray tube, thus affecting the focal spot size.
Particularly, while operating the X-ray tube with a low electron beam current, such as about 10 milliampere (mA) and 140 kilovolts (kV) the strong influence of the electro-magnetic forces overly focus the electron beam to form a constricted “waist” in the electron beam trajectory. Reversing this narrowing effect in the electron beam during imaging is a challenging task. The narrowing effect hinders the ability of the X-ray tube to precisely control the positioning and the focusing of the electron beam at a target location at low electron beam currents, thus impeding imaging system performance.
It is desirable to develop effective methods and systems that enable an electron beam system of an X-ray tube to operate in a wide dynamic range of emission. Particularly, there is a need for an electron beam system that controls the electron beam intensity to accurately position the electron beam at a target location based on imaging requirements. Further, it is also be desirable to develop methods and systems that control focus and position of the electron beam to achieve robust imaging system performance while preserving image quality and durability of the X-ray source.